1. Field of the Invention
The present invention relates to an ink-jet printing apparatus and a nozzle member used in the same and, more particularly, to a film nozzle member having a plurality of holes for holding ink and an ink-jet printing apparatus wherein the ink in the holes is selectively heated by heating elements formed on an upper end portion of a thermal printing head while the nozzle member is moved on the thermal head, the ink is ejected from the holes by the pressure of bubbles formed adjacently to the heating elements in the heated ink, and the ejected ink is attached to a recording medium which is being conveyed in the vicinity of the thermal head and close to the nozzle member, thereby performing printing.
2. Description of the Related Art
Conventionally, non-impact printing methods including an electrostatic printing method, a thermal printing method, a thermal transfer printing method, an ink-jet printing method, etc. and the like have been proposed.
Among them, the ink-jet printing method is advantageous in that it generates low noise, requires low power, can be made compact, is easily applicable to color printing, and can perform relatively high-speed printing.
The ink-jet printing method is roughly classified into a continuous jet-type, intermittent jet-type, on-demand type, and ink mist type ink-jet by the differences in the arrangements of the ink droplet generating means. The on-demand type ink-jet printing method is the main trend in these days due to a simple arrangement. As the on-demand type ink-jet printing method, a Gould method and a bubble jet method are used in practice. In the Gould method, ink droplets are ejected from a tubular nozzle member by radial contraction of a cylindrical piezoelectric transducer arranged to surround the nozzle member. In the bubble jet method, ink droplets are ejected from a tubular nozzle member by the pressure of bubbles that are formed adjacently to the heating elements in the ink in the nozzle number by instantaneously heating elements in the nozzle member.
However, in either the Gould or bubble jet method, a single tubular nozzle member having a very small diameter corresponds to a single piezoelectric transducer or a heating element. Therefore, ink tends to clog in the nozzle member if the ink-jet apparatus is not used for a long period of time, or bubble retention in the nozzle member tends to occur immediately after ink ejection. In the Gould method, the integration density of tubular nozzle members is comparatively lower than the integration density of printing dot forming means of various other non-impact printing apparatuses. Therefore, the resolution of a printed character, figure, and other symbols is not higher than that of the various other non-impact printing apparatuses. Although the printing speed of a Gould or bubble jet-method non-impact serial printing apparatus is higher than that of various other non-impact serial printing apparatuses, it is lower than that of a non-impact line printing apparatus of another method. This is because it is difficult to increase the integration density of nozzle members.
A non-impact ink-jet printing apparatus disclosed in Japanese Pat. Disclosure (Kokai) No. 60-71260 which is a base application for U.S. Pat. No. 4,608,577 is developed to eliminate the drawbacks of the conventional non-impact ink-jet printing apparatuses.
The novel ink-jet printing apparatus uses as a nozzle member a film which is made of a metal, a heat-resistant synthetic resin, or a multilayer member of the metal and resin, and has a plurality of holes with a diameter of 10 to 200 .mu.m. The film nozzle member is slid on the heating elements of the thermal printing head while ink is filled in the holes. Bubbles are generated adjacently to the heating elements in the ink rapidly heated by the heating elements, and the pressure of the bubbles ejects ink droplets from the holes. The ejected ink droplets are attached to the recording medium (normally a recording paper), which is being conveyed close to the film nozzle member in the vicinity of the thermal printing head, and form ink dots. A character, figure, or other symbols is printed by a group of ink dots.
In the novel ink-jet printing apparatus described above, since long and narrow tubular nozzle members are not used, ink clogging or bubble retention in the nozzle members does not occur in principle. The integration density of the holes can be increased to be considerably higher than that of small-diameter tubular nozzle members described above, and these holes can be formed in a relatively large area at a high density. Therefore, when such a film nozzle member haviing a plurality of holes formed in a relatively large area at a high density is combined with a thermal printing head having a plurality of heating elements, high-resolution, high-speed printing is enabled.
However, in order to obtain a sufficient printing density with the novel ink-jet printing apparatus, the film nozzle member filled with ink in its holes must be conveyed at least the same speed as the conveying speed of the recording medium. During printing, nonused holes (i.e., a hole sufficiently filled with the ink) must constantly be supplied to heating elements of the thermal printing head. Accordingly, the film nozzle member must have a length larger than that of the recording medium. When a film nozzle member having such a length and a thermal printing head are combined as a detachable cartridge, the cartridge becomes comparatively large and the entire printing apparatus cannot be made compact. Also, the film nozzle member and the thermal printing head wear within a short period of time, resulting in a short service life.
When the conveying speed of the film nozzle member is decreased to be lower than that of the recording medium, the length of the film nozzle member can be decreased. As a result, the cartridge and then the entire printing apparatus can be made compact, and progress of wear of the film nozzle member and the thermal printing head can be considerably delayed, resulting in a considerably long service life. In this case, however, if the same heating element is consecutively heated, a hole which contains no ink or only a small amount of ink upon the former ink ejection is heated again before it is refilled with ink. When such a hole is heated, it ejects substantially no ink, and the printing density is decreased.
In the novel ink-jet printing apparatus described above, when the urging force of the film nozzle member against the thermal printing head is excessively large, an ink for refilling that must be retained between the thermal printing head and the film nozzle member is squeezed out from the gap between them. Therefore, even when the same heating element is not consecutively heated, the holes which contains no ink or only a small amount of ink upon the former ink ejection cannot be sufficiently refilled with ink, resulting in a decrease in printing density.